Position-loop based cross-coupled and synchronization control of a parallel kinematics machine

Compared with the tracking error, the contouring error has a greater impact on product quality. This paper is concerned with the contouring control of the parallel robots, whose kinematics is nonlinear, so it's hard to utilize the conventional cross-coupled control method directly. Thereby, the position loop-based cross-coupled control is applied. In this method, the contouring error is estimated as the minimum distance from the actual position to the approximated circle. The position-reference input are modified to implement the contouring error compensation. In addition, to strengthen the coordination of the two actual axes further, we proposed to add the synchronization controller to the position loop-based cross-coupled control structure. For the planar motion, the synchronization error of workspace is defined as the difference value between the tracking error of x axis and the tracking error of y axis. It is compensated in the joint space of the driven axes instead of the workspace to improve the motion accuracy more directly. A number of experiments are carried out for circular contour at different speed. The results show the proposed method really leads to improved contouring accuracy.